1. Field of the Invention
This invention relates to the reforming of hydrocarbons to make fuel gases such as gases containing a high content of methane. More particularly the invention relates to catalyst for use in the reforming of hydrocarbons, processes for the manufacture of such catalysts and the processes for the production of methane-containing gases employing such catalysts.
2. The Prior Art
The catalytic reforming of hydrocarbons with steam has been known for many years and was first developed for the production of hydrogen-rich gases. Such processes were carried out at high temperatures, e.g., about 700.degree. C. The catalysts employed in such processes include nickel-alumina based catalysts in which the alumina was a resistant refractory material since the catalyst had to be able to withstand high temperatures. With light feedstocks such as methane it was possible to employ commercially practicable amounts of steam at high temperatures to avoid the problem of carbon deposition on the catalyst. However, as heavier feedstocks were used the excess of steam required to prevent carbon deposition rendered the process less commercially feasible with the existing nickel-alumina catalysts. In the late 1950's, modified catalysts were developed which were claimed to operate effectively at significantly lower steam-to-hydrocarbon ratios than those required for the existing nickel-alumina catalysts. These modified catalysts contained from 0.5 to 30% by weight of the catalyst of an alkali metal. Such catalysts are described in U.K. Patent Specification Nos. 966882 and 966883 in the counterpart U.S. Pat. No. 3,119,667.
It was also known that methane-containing gases could be produced by effecting the Sabatier Reaction on gases containing hydrogen and carbon monoxide and that such hydrogen and carbon monoxide-containing gases could be produced by the high temperature reforming of higher hydrocarbons with steam.
The methane-containing gas was generally suitable as a 500 BTU Towns Gas. However, this combination of a first stage which was both endothermic and required high temperatures with an exothermic, lower temperature second stage led to difficulties in heat recovery and made efficient and economic production difficult to achieve.
In the mid 1950s the art of making methane-containing gas was considerably advanced by the advent of the Catalytic Rich Gas or CRG Process. This process was a steam reforming reaction of higher hydrocarbons over nickel-alumina catalysts at lower temperatures than those required for hydrogen production. In addition to the advantages that the use of low temperatures favored increased concentration of methane in the equilibrium product gas, the pressures employed enabled the gases to be transmitted and distributed without pumping facilities, and furthermore under the conditions employed the reaction could be performed in adiabatic reactors. This process was first described in U.K. Patent Specification No. 820257. In the early days of the CRG process the catalysts were usually of low nickel content, e.g., of the order of 15% by weight. The increased activity necessary for low temperature operation was achieved by coprecipitating the nickel and alumina components. The coprecipitation of the nickel and alumina components resulted in the production of catalytically active transitional aluminas having high surface areas. Concomittantly, it was possible to achieve better distribution of the nickel over the support. Thus the structure of the catalysts differed considerably from those proposed both earlier and later for hydrogen production.
The CRG process has been considerably developed in view of the necessity of using heavier and heavier feedstocks and with the need to produce a substitute natural gas (SNG) to augment or replace depleted supplies of natural gas. The development of the basic CRG process and supplemental or ancilliary techniques has been described for example in U.K. Patent Specification Nos. 969637, 994278, 1150066, 1152009, 1155843 and 1265481 and in U.S. Pat. Nos. 3,415,634, 3,410,642, 3,433,609, 3,441,395, 3,459,520, 3,469,957, 3,511,624, 3,515,527, 3,625,665 and 3,642,460. These modified processes are themselves very efficient, but the governing constraint is the question of maintaining catalyst activity under the reforming conditions. Under conditions of high temperature reforming the main problem is to prevent carbon deposition by cracking or from the Boudouard Reaction. As recognised in the prior art, this problem can be alleviated by the use of excess steam. However, with heavier feedstocks the steam requirement becomes uneconomically excessive and it has been necessary to use alkali-promoted catalysts of the type described above. With low temperature reforming a problem associated with catalyst performance is a loss of activity owing to the deposition of polymeric substances on the catalyst surface.
In the early days of CRG operation polymer deposition was not so significant because relatively light feedstocks were available and working pressures required for Towns Gas production were relatively low. More recently world conditions have required the process to be available for heavier and more aromatic feedstocks and especially feedstocks other than `straight run` feedstocks. The use of such feedstocks increases the risk of polymer formation.
The problems of polymer deposition were considered in the 1960s and in this respect the Specification of U.K. Pat. Nos. 969637 and 1150066 are addressed to this problem. Accordingly, it has been proposed to include an alkali and/or alkaline earth metal compound, preferably in amounts of from 0.75 to 8.6%, to overcome the problem of polymer deposition. Such additions were proposed for catalysts working conventionally at pressures of from 10 to 25 atmospheres, i.e., relatively low pressures in CRG terms. It was found that although such additions benefited catalyst life as far as polymer deposition was concerned, the overall catalyst life was not as great as might have been expected. It was observed that polymer formation was a functiion of temperature and that higher preheat temperatures increased the catalyst's resistance to polymer deactivation. However, since the alumina components of the catalyst were transitional aluminas, the catalyst had a reduced resistance to sintering. It was further observed that the presence of the alkali had no beneficial, or even deleterious, effect on the sinter resistance of the catalyst, and may even have a deleterious effect.
The recognition of this problem and the proposals for catalyst formulation to meet the need for catalysts having resistance to deactivation by both polymer formation and sintering are described in our U.K. Patent Specification No. 1150066.
In that Specification it is proposed that sintering could be significantly reduced and adequate resistance to polymer deposition achieved by reforming light hydrocarbons using a catalyst having alkali contents from 0.1 to 0.75%. Alkali values of 0.4 to 0.7 were preferred to achieve the optimum of minimum for polymer deposition resistance consistent with the maximum for sinter resistance. Thus, it was possible to produce catalysts having adequate polymer and sinter resistance. However, since the presence of alkali had both a beneficial and an adverse effect on the catalyst performance, it remained essential to carefully control the reaction temperature since the temperature at the inlet of the reactor had to be high enough to guard against severe polymer forming conditions and the temperature at the outlet low enough to prevent sintering.
Although Specification No. 1150066 discloses a broad range of alkali promoter, i.e., from 0.10 to 0.75 and operating at pressures up to 50 atmospheres, it is clear that such catalyst exhibiting the best performance had alkali contents of from 0.4 to 0.7% and were employed in steam reforming reactions at pressures ranging from 10 to 25 atmospheres.
Since the use of lighter feedstocks required but fairly low inlet temperatures to combat polymer deactivation, sintering problems were not encountered and the overall performance was generally satisfactory. However, when heavier feedstocks began to be used, increased inlet temperatures were required to counteract polymer formation and this in turn led to increased tendency to sintering and unsatisfactory life performance. The sintering problem with heavier feedstocks was accentuated at the higher pressures required for the production of SNG, such higher pressures favouring increased methane formation in the product gas.